SUNDAY, April 13 (HealthDay News) -- Alcohol, consumed even in small amounts, increases the risk of breast cancer and particularly estrogen-receptor and progesterone-receptor positive breast cancer, a new study shows.

The findings, expected to be presented Sunday at the annual meeting of the American Association for Cancer Research, in San Diego, are followed by a second study that found an association between breast cancer risk and two genes involved in alcohol metabolism.

Previous data has suggested that consuming alcohol ups the risk of breast cancer, although the precise mechanisms have not been clarified.

In some forms of breast cancer, malignant cells have receptors that render them sensitive to hormones such as estrogen. The first study aimed to see if the hormone receptor status of the tumor influenced the relationship between alcohol consumption and breast cancer risk.

In the study, a team led by Dr. Jasmine Lew of the U.S. National Cancer Institute followed more than 184,000 postmenopausal women for an average of seven years.

Those who had less than one drink a day had a 7 percent increased risk of breast cancer compared to teetotalers, the team reported. Women who drank one to two drinks a day had a 32 percent increased risk, and those who had three or more glasses of alcohol a day had up to a 51 percent increased risk.

But the risk was seen mostly in those 70 percent of tumors classified as estrogen receptor- and progesterone receptor-positive. Researchers suspect that alcohol may have an effect on breast cancer via an effect on estrogen.

The risk was similar whether women consumed primarily beer, wine or spirits, the NCI team noted.

The second study dug deeper into other possible mechanism by which alcohol consumption increases breast cancer risk.

"For years, we've known that there's an association between alcohol drinking and breast cancer risk, but nobody knows yet what the underlying biological mechanisms are," said Dr. Catalin Marian, lead author of the study and a research instructor in oncology at the Lombardi Comprehensive Cancer Center at Georgetown University in Washington, D.C. "The logical step was to begin analyzing the alcohol metabolizing genes."

And indeed, two of these genes -- ADH1B and ADH1C -- were associated with a two-fold increase in breast cancer risk.

But the study does not prove a definite cause-and-effect link. "This is an association," Marian said. "This type of study is good for generating hypotheses. It's not a definite conclusion. It needs to be replicated by other studies to say for sure that what we found is there."

Another researcher urged caution in interpreting the results of both studies.

"These studies are too early for use in a clinical setting or to advance a public health message," said Dr. Peter Shields, co-author of the genetics study and deputy director of the Lombardi Comprehensive Cancer Center.